Access circuit and method for allowing external test voltage to be applied to isolated wells

ABSTRACT

An access circuit selectively couples an externally accessible terminal to each of a plurality of isolated DRAM wells in which respective DRAM arrays are fabricated. The access circuit for each well includes first and second transistors fabricated in respective wells coupled between the externally accessible terminal and a respective one of the DRAM wells. The well of the first transistor is coupled to the externally accessible terminal, and the well of the other transistor is coupled to a respective DRAM well. A control circuit applies select signals to gate electrodes of the first and second transistors. The control circuit includes respective shunt transistors that shunt the gate electrodes to the source regions of the first and second transistors when the transistors are turned off to isolate the respective DRAM wells from the external terminal regardless of the magnitude and polarity of a test voltage applied to the externally accessible terminal.

TECHNICAL FIELD

This invention relates to integrated circuits such as memory devices,and, more particularly, to a circuit and method for allowing anexternally applied test voltage to be applied to each of severalisolated wells in which semiconductor devices are fabricated.

BACKGROUND OF THE INVENTION

With reference to FIG. 1, a semiconductor structure 10 includes asubstrate 12 having formed therein a well 14, which is a region of thesubstrate 12 that is doped oppositely to the doping of the substrate 12.For example, the substrate 12 of FIG. 1 is a p-type substrate. A deepn-type well 16 is implanted into the substrate 12 to form a buriedlayer. A surface n-well in areas 18, 20 surrounds the buried layern-type well 16 to form a doughnut shape where the center is a p-typewell 14 that is now completely isolated from the substrate 12, thusallowing a separate bias voltage. Various semiconductor devices can befabricated into the well 14, such as an n-channel transistor 22. Inpractice, a large number of semiconductor devices can be fabricated intothe well 14 to form an integrated circuit.

The junction between the well 14 and the deep n-well 16 forms a p-njunction or diode 24 that is schematically illustrated in phantom inFIG. 1. A similar diode 26 is formed by a p-n junction between the deepn-well 16 and the p-type substrate 12. The diodes 24, 26 are inherentlyformed with a back-to-back connection, thereby isolating the p-well 14from the substrate 12, and, therefore, from other devices that are alsofabricated into the substrate 12. The p-well 14 is often biased to anegative bias voltage, which is often provided by a negative voltagecharge pump, commonly known as a V_(bb) charge pump. By fabricating thememory cell arrays in respective wells, each of several array wells canbe isolated from each other and from other circuitry fabricated in thesubstrate 12. Similar diodes (not shown) are formed by the p-n junctionsbetween the n-wells 18, 20 and both the p-well 14 and the substrate 12.However, the diodes are inherently coupled to each other back-to-back toisolate the p-well from the substrate 12. In practice, the n-wells 18,20 are typically biased to a relatively large positive voltage, such asa supply voltage V_(CC) or a positive pumped voltage V_(CCP), tomaintain the diodes reverse or back-bias.

As explained above, fabricating integrated circuits into the wells alsoallows the wells 14 to be biased to a voltage that enhances theperformance of integrated circuits fabricated in the wells 14. Forexample, it is common to bias the wells 14 in which DRAM arrays havebeen fabricated to a negative voltage, which reduces the leakage ofaccess transistors (not shown) coupled to respective DRAM memory cells.As is well known in the art, reducing access transistor leakage allowsthe memory cells to store data for a longer period to reduce refreshrates. However, it is desirable to perform post-fabrication testing ofthe integrated circuit under “worst case” conditions for the purpose ofdetecting failures that are likely to occur after the integrated circuitis placed in service. Again using the example of DRAM arrays, it isdesirable to test the data retention time of a DRAM array fabricated inthe well 14 with the well 14 biased to ground voltage. If the dataretention time is achieved with the well 14 biased to ground or a lessnegative voltage, then it will inherently do so when the well 14 isbiased to a more negative voltage during operation. The test voltage canbe applied to the well 14 from an externally accessible terminal 30. Toallow the terminal 30 to be used for other purposes during use of anintegrated circuit fabricated in the substrate 12, the terminal 30 ispreferably coupled to the well 14 through a transistor 34 fabricatedinto the substrate 12.

FIG. 2 shows a semiconductor structure 40 in which a p-type well or core42 is fabricated in a substrate 44, and a DRAM array is 46 fabricated inthe p-type core 42. Like the p-well 14 of FIG. 1, the core 42 isisolated from the substrate 12 by n-wells 18, 20 (not shown in FIG. 2)and the deep n-well 16. The core 42 is coupled through a p+ region 47 toan externally accessible terminal 48 through an n-channel transistor 50.The transistor 50 is fabricated in a p-type well 52 in which first andsecond n-type source-drain regions 56, 58 are fabricated. The p-well 52is also isolated from the substrate 12 by n-wells 18, 20 and the deepn-well 16. A gate electrode 60 is fabricated between the source-drainregions 56, 58 and is insulated from the well 42 by a gate insulatinglayer 62 to form an MOS device. The externally accessible terminal 48 isconnected to the first source-drain region 56 The second source-drainregion 58 is connected to the well 52 through a p+ region 54 that ismore positively doped than the doping of the p-well 52 and to the core42 in which the array 46 is fabricated through a similar p+ region 55.As is well-known in the art, coupling the terminal 48 to the well 52 andto the core 42 through p+ regions 54, 55, respectively, reduces theresistance of the contact with the well 52 and to the core 42. Asexplained above with reference to FIG. 1, the junction between eachp-type region and each n-type region forms a diode, one of which 68 isshown in phantom in FIG. 2. The significance of this diode 68 will beexplained with reference to FIG. 3. There is also a diode created by thejunction between the n-type source-drain region 58 and the p-type well52, but since this diode would be shorted out by the direct connectionbetween the region 58 and well 52, it is not shown in FIG. 2.

In operation, the transistor 50 isolates the externally accessibleterminal 48 from the core 42 during normal use of an integrated circuit,such as a DRAM device, fabricated in the substrate 44. During this time,the core 42 can be biased to a negative voltage while the substrate 44remains at ground potential. When the array 46 is to be tested under“worst-case” conditions, a positive voltage is applied to the gateelectrode 60 by suitable means to form a conductive n-type channelbetween the source-drain regions 56, 58. The transistor 50 then couplesthe externally accessible terminal 48 to the core 42 so that a positivetest voltage can be applied to the core 42 through the terminal 48. Anegative test voltage can also be applied to the core 42 through theterminal 48 when the transistor 50 is turned ON. However, if thenegative test voltage is more than about 0.6 v less than the voltage towhich the core 42 is biased by a V_(bb) charge pump (not shown in FIG.2), the diode 68 will be forward biased. As a result, the core 42 willbe coupled to the externally accessible terminal 48 regardless of theconductive state of the transistor 50.

The problem caused by the diode 68 becoming forward-biased by a negativetest voltage will now be explained with reference to FIG. 3. FIG. 3shows a portion of a DRAM device 70 having two DRAM cores 72, 74 coupledto respective V_(bb) charge pumps 76, 78. The cores 72, 74 are alsocoupled to an externally accessible terminal 80 by respectivetransistors 82, 84, each of which is controlled by a respective selectsignal, Sel. A and Sel. B, respectively. The diodes 68 coupled betweenthe source-drain regions 56, 58 (FIG. 2) of the transistors 82, 84 arealso shown in FIG. 3. Although the DRAM device 70 is shown in FIG. 3 ashaving two DRAM cores 72, 74, it will be understood that other DRAMdevices can have a large number of DRAM cores.

During testing of the DRAM device 70, it is desirable to individuallytest the operation of each of the cores 72, 74 without affecting theoperation of the core not being tested. For example, it is desirable tobe able to apply a negative or positive test voltage to the terminal 80,and then individually turn ON each of the transistors 82, 84 to couplethe test voltages to the respective cores 72, 74. However, when a testvoltage is applied to the terminal 80 that is significantly morenegative than the bias voltages being supplied to the cores 72, 74 bythe V_(bb) charge pumps 76, 78, the OFF transistor 82, 84 willeffectively be rendered conductive by the diode 68 becomingforward-biased. For example, when the Sel. B signal is driven high toturn ON the transistor 84, a large negative test voltage can be appliedto the core 74 from the terminal 80. However, this test voltage willalso be applied to the core 72 through the diode 68 formed in parallelwith the transistor 82. For this reason, it is not possible toadequately test the cores of an integrated circuit using the circuitryshown in FIG. 2, since a large negative voltage cannot be applied to theindividual cores. There is therefore a need for a circuit and methodthat would allow integrated circuit cores to be individually tested withrelatively large negative and positive test voltages.

SUMMARY OF THE INVENTION

An access circuit and method is used to apply a test voltage from anexternally accessible terminal to each of a plurality of isolatedcircuit wells fabricated in a semiconductor substrate. The accesscircuit and method includes a first transistor fabricated in a firstwell formed in the semiconductor substrate that is isolated from thecircuit wells. The first transistor has a first source-drain regionfabricated in the first well that is coupled to the externallyaccessible terminal and to the first well. The first transistor alsoincludes a second source-drain region fabricated in the first well, anda gate electrode fabricated between the first and second source-drainregions. The access circuit also includes a second transistor that maybe fabricated in a second well formed in the semiconductor substratethat is isolated from the circuit wells and the first well. The secondtransistor has a first source-drain region fabricated in the second wellthat is coupled to the second source-drain region of the firsttransistor. A second source-drain region fabricated in the second wellis coupled to the second well and to a respective one of the circuitwells. The second transistor also includes a gate electrode fabricatedbetween the first and second source-drain regions in the second well.

The access circuit and method may include a control circuit for applyingselect signals to the gate electrodes of each of the first and secondtransistors. The control circuit preferably includes for each of thefirst and second transistors a shunt transistor and a controltransistor. The shunt transistor is coupled between the externallyaccessible terminal and the gate electrode of the first transistor. Thecontrol transistor is coupled between the externally accessible terminaland a gate of the shunt transistor, and it has a gate coupled to receivean access signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a semiconductor structure showing anisolated well fabricated in a substrate and coupled to an externallyaccessible terminal in a conventional manner.

FIG. 2 is another cross-sectional view of a semiconductor structureshowing an isolated well fabricated in a substrate and coupled to anexternally accessible terminal in a conventional manner.

FIG. 3 is a schematic and block diagram of a conventional circuit forcoupling an externally accessible terminal to either of two DRAM coresusing the semiconductor structure of FIG. 2.

FIG. 4 is a cross-sectional view of a semiconductor structure that canbe used to couple an externally accessible terminal to either of the twoDRAM cores according to one embodiment of the invention.

FIG. 5 is a schematic and block diagram of an externally accessibleterminal coupled to either of two DRAM cores using the semiconductorstructure of FIGS. 3 and 4.

FIG. 6 is a schematic and logic diagram of a control circuit that can beused to generate control signals for use by the semiconductor structureof FIGS. 4 and 5.

FIG. 7 is a block diagram of one embodiment of a memory device using thesemiconductor structure of FIGS. 4 and 5 and the control circuit of FIG.6, or a semiconductor structure and/or control circuit according to someother embodiment of the invention.

FIG. 8 is a block diagram of one embodiment of a computer system usingthe memory device of FIG. 7 or some other embodiment of a memory devicein accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 4 shows a semiconductor structure 100 that can be used in place ofeach of the transistors 82, 84 used in the DRAM device 70 shown in FIG.3. The semiconductor structure 100 includes a first transistor 104coupled in series with a second transistor 106. The transistors 104, 106are coupled between the externally accessible terminal 80 (FIG. 3) andrespective one of the DRAM cores 72, 74. The transistors 104, 106 arefabricated in respective wells 114, 116 that are, in turn, fabricated ina substrate 108. Like the p-well 14 of FIG. 1, the wells 114, 116 areisolated from the substrate 108 by a pair of n-wells 18, 20 and the deepn-well 16. In the embodiment shown in FIG. 4, the substrate 108 is ap-type substrate, and the wells 114, 116 are also p-type wells. Thefirst transistor 104 includes a first n-type source-drain region 120that is coupled to the externally accessible terminal 110 and to thewell 114 through a p+ region 115 that is more positively doped than thedoping of the p-well 114. A second n-type source-drain region 122 isfabricated in the well 114 and is separated from the first source-drainregion 120 by a gate electrode 126 isolated from the well 114 by a gateinsulating layer 128. The gate electrode 126 is coupled to receive oneof the select signals, Sel. 1 or Sel. 2.

The second transistor 106 similarly includes a first n-type source-drainregion 130 fabricated in the well 116, a second n-type source-drainregion 132 fabricated in the well 116, and a gate electrode 136positioned between the source-drain regions 130, 132. The gate electrode136 is isolated from the well 116 by a gate insulating layer 138. Thefirst source-drain region 130 of the second transistor 106 is coupled tothe source-drain region 122 of the first transistor 104, and the secondsource-drain region 132 of the second transistor 106 is coupled to oneof the cores 72, 74 and to the well 116 through a p+ region 117 that ismore positively doped than the doping of the p-well 116. The gateelectrode 136 is also coupled to the gate electrode 126 of the firsttransistor 104 so that it also receives one of the select signals, Sel.1 or Sel. 2. However, the gate electrodes 126, 136 of the respectivetransistors 104, 106 may alternatively be coupled to respective controlsignals as long as they both have the appropriate logic levels at theproper times.

As with the semiconductor structure of FIG. 2, respective diodes 140,142 shown in phantom in FIG. 4 are inherently formed by the junctionbetween the source-drain region 122 and the well 114 and by the junctionbetween the source-drain region 130 and the well 116. Respective diodes140, 142 are also inherently formed by the junction between thesource-drain region 120 and the well 114, and by the junction betweenthe source-drain region 132 and the well 116, but these are shunted bythe direct connection between the source-drain regions 120, 132 and thewells 114, 116, respectively.

The operation of the semiconductor structure 100 will now be explainedwith reference to the schematic of an access circuit 144 shown in FIG. 5that uses the structure 100. The transistors 104, 106 are turned ON toprovide the externally accessible terminal 80 with access to one of twooutput terminals 146, 148 responsive to a high Sel. A or Sel. B signal,respectively. The output nodes 146, 148 are coupled to the DRAM cores72, 74, respectively. The select signals Sel. A and Sel. B are providedby a control circuit 150, an embodiment of which will be explained withreference to FIG. 6. When one of the Sel signals is low, the respectiveone of the transistors 104, 106 is turned OFF to isolate the respectivecore 72, 74 from the externally accessible terminal 80. It is importantto note that the semiconductor structure 100 shown in FIG. 4 isfabricated so that the diodes 140, 142 are coupled to each other“back-to-back.” As a result, one of the diodes 140, 142 will isolate thecore 72 or 74 to which it is coupled from the terminal 80 regardless ofthe magnitude and polarity of the voltage applied to the terminal 80 aslong as the voltage is not large enough to cause the diodes 140, 142 togo into an avalanche breakdown mode. Thus, unlike the semiconductorstructure 40 shown in FIG. 2, a test voltage can be applied to one ofthe cores 72, 74 but not to the other regardless of the magnitude andpolarity of the test voltage.

One embodiment of the control circuit 150 that can be used to generatethe Sel. 1 and Sel. 2 signals is shown in FIG. 6. The control circuit150 a used to generate the Sel. 1 signal is shown in detail in FIG. 6,it being understood that the control circuit 150 b used to generate theSel 2 signal is identical to the circuit 150 a. The control circuit 150a includes a decode circuit 154 that includes two NAND gates 160, 162and one inverter 164. The NAND gate 160 is enabled to generate an activelow ACCESS* signal whenever a PWRUP signal is active high, whichnormally occurs when the cores 72, 74 are operational. The PWRUP signalis generated by coupling a global PWRUP signal through two inverters166, 168. The NAND gate generates the active low ACCESS* signal whenevereither a VBBACC1* signal is active low 6 r PRBMD signal is high and thuscauses the inverter 164 to apply a low to the NAND gate 160. The PRBMDsignal is generated by coupling a global PRBMD signal through twoinverters 170, 172. The VBBACC1* signal, which is generated by couplinga VBBACC1 signal through an inverter 176, is not a global signal and cantherefore be used to individually enable the control circuit 150 a. Aseparate access signal, VBBACC2, is coupled through an inverter 178 tosupply an active low VBBACC2* signal to the other control circuit 150 b.

The ACCESS* signal is applied to two identical drive circuits 180 a,b,one of which 180 a is coupled to the gate electrode 126 (FIG. 4) tocontrol the conductive state of the first transistor 104 and the otherof which 180 b is coupled to the gate electrode 136 to control theconductive state of the second transistor 106. In the interest ofbrevity and clarity, an explanation of only the drive circuit 180 a willbe provided, it being understood that the drive circuit 180 b operatesin the same manner. When the ACCESS* signal is active low, it turns ON aPMOS transistor 184 thereby coupling a power supply voltage V_(CC) tothe gate electrode 126 of the first transistor 104. As previouslyexplained, the first transistor 104 then couples the externallyaccessible terminal 80 to the second transistor 106. Since both controlcircuits 180 a,b operate in the same manner, the active low ACCESS*signal also causes the control circuit 180 b to apply V_(CC) to the gateelectrode 136 of the second transistor 106, thereby coupling theexternally accessible terminal 80 to the output node 146. The controlcircuit 150 b operates in substantially the same manner except that itis enabled by the VBBACC2 signal. As a result, the externally accessibleterminal 180 is normally coupled to either the output node 146 or theoutput node 148, but not both output nodes at the same time.

The voltage V_(CC) coupled through the PMOS transistor 184 is alsoapplied to the gate of an NMOS transistor 188 to turn ON the transistor188. The transistor 188 is coupled to the gate of an NMOS shuntingtransistor 190, which is held OFF by the ON transistor 188. The activelow ACCESS* signal is also applied to an inverter 194, which thenoutputs a high to turn OFF a PMOS transistor 198.

When the ACCESS* signal transitions to an inactive high level, it turnsOFF the PMOS transistor 184, which subsequently allows the transistor188 to turn OFF since it no longer couples V_(CC) to the gate of thetransistor 188. The high ACCESS* signal also causes the inverter 194 tooutput a low, which turns on the PMOS transistor 198. The transistor 198then couples V_(CC) to the gate of the shunting transistor 190, therebyturning the transistor 190 ON. The ON shunting transistor 190 shunts thegate electrode 126 of the first transistor 104 to its source, therebyholding the transistor 190 OFF regardless of what voltage is applied tothe externally accessible terminal 80. The ON shunting transistor alsoholds the transistor 188 in an OFF condition.

As mentioned above, the other control circuit 180 b operates in the samemanner as the control circuit 180 a to hold the second transistor 106OFF responsive to the inactive high ACCESS* signal.

FIG. 7 is a block diagram of a conventional synchronous dynamic randomaccess memory (“SDRAM”) 200 that utilizes the access circuit shown inFIGS. 4-6 or some other embodiment of the invention, although variousembodiment of the invention can also be used with memory devices otherthan the SDRAM 200 and devices other than memory devices. The operationof the SDRAM 200 is controlled by a command decoder 204 responsive tohigh-level command signals received on a control bus 206 and coupledthorough input receivers 208. These high level command signals, whichare typically generated by a memory controller (not shown in FIG. 7),are a clock enable signal CKE*, a clock signal CLK, a chip select signalCS*, a write enable signal WE*, a row address strobe signal RAS*, acolumn address strobe signal CAS*, and a data mask signal DQM, in whichthe “*” designates the signal as active low. The command decoder 204generates a sequence of command signals responsive to the high levelcommand signals to carry out the function (e.g. a read or a write)designated by each of the high level command signals. These commandsignals, and the manner in which they accomplish their respectivefunctions, are conventional. Therefore, in the interest of brevity, afurther explanation of these command signals will be omitted.

The SDRAM 200 includes an address register 212 that receives rowaddresses and column addresses through an address bus 214. The addressbus 214 is generally coupled through input receivers 210 and thenapplied to a memory controller (not shown in FIG. 7). A row address isgenerally first received by the address register 212 and applied to arow address multiplexer 218. The row address multiplexer 218 couples therow address to a number of components associated with either of twomemory banks 220, 222 depending upon the state of a bank address bitforming part of the row address. As previously explained, each of thememory banks 220, 222 is fabricated in respective DRAM cores 72, 74 thatare normally biased to a negative voltage by respective V_(bb) chargepumps 76, 78. Associated with each of the memory banks 220, 222 is arespective row address latch 226, which stores the row address, and arow decoder 228, which decodes the row address and applies correspondingsignals to one of the arrays 220 or 222. The row address multiplexer 218also couples row addresses to the row address latches 226 for thepurpose of refreshing the memory cells in the arrays 220, 222. The rowaddresses are generated for refresh purposes by a refresh counter 230,which is controlled by a refresh controller 232. The refresh controller232 is, in turn, controlled by the command decoder 204.

In accordance with one embodiment of the invention, the lowest-orderedaddress bit of the address bus 214 is applied to an externallyaccessible terminal that is coupled through the access circuit 144 tothe cores 72, 74 for the respective arrays 220, 222. The SDRAM 200includes conventional circuitry to generate the global PWRUP and PRBMDsignals and the individual VBBACC1 and VBBACC2 signals discussed abovewith reference to FIG. 6. However, it will be understood that otherterminals of the SDRAM 200 may be used as the externally accessibleterminal for the access circuit 144, including terminals dedicated tothat function. Also, the PWRUP, PRBMD, VBBACC1 and VBBACC2 signals maybe generated externally as well as internally. When the externallyaccessible terminal is coupled to one or both of the cores 72, 74, itmay be desirable to disable the charge pump 78 by generating an activelow signal C2* whenever an active low ACCESS* signal is generated ineither of the control circuits 150 a,b.

After the row address has been applied to the address register 212 andstored in one of the row address latches 226, a column address isapplied to the address register 212. The address register 212 couplesthe column address to a column address latch 240. Depending on theoperating mode of the SDRAM 200, the column address is either coupledthrough a burst counter 242 to a column address buffer 244, or to theburst counter 242 which applies a sequence of column addresses to thecolumn address buffer 244 starting at the column address output by theaddress register 212. In either case, the column address buffer 244applies a column address to a column decoder 248.

Data to be read from one of the arrays 220, 222 is coupled to the columncircuitry 254, 255 for one of the arrays 220, 222, respectively. Thedata is then coupled through a data output register 256 and data outputdrivers 257 to a data bus 258. Data to be written to one of the arrays220, 222 are coupled from the data bus 258 through a data inputreceivers 259 to a data input register 260. The write data are coupledto the column circuitry 254, 255 where they are transferred to one ofthe arrays 220, 222, respectively. A mask register 264 responds to adata mask DM signal to selectively alter the flow of data into and outof the column circuitry 254, 255, such as by selectively masking data tobe read from the arrays 220, 222.

FIG. 8 shows an embodiment of a computer system 300 that may use theSDRAM 200 or some other memory device that used one of the embodimentsof an access circuit described above or some other embodiment of theinvention. The computer system 300 includes a processor 302 forperforming various computing functions, such as executing specificsoftware to perform specific calculations or tasks. The processor 302includes a processor bus 304 that normally includes an address bus, acontrol bus, and a data bus. In addition, the computer system 300includes one or more input devices 314, such as a keyboard or a mouse,coupled to the processor 302 to allow an operator to interface with thecomputer system 300. Typically, the computer system 300 also includesone or more output devices 316 coupled to the processor 302, such outputdevices typically being a printer or a video terminal. One or more datastorage devices 518 are also typically coupled to the processor 302 tostore data or retrieve data from external storage media (not shown).Examples of typical storage devices 318 include hard and floppy disks,tape cassettes, and compact disk read-only memories (CD-ROMs). Theprocessor 302 is also typically coupled to a cache memory 326, which isusually static random access memory (“SRAM”) and to the SDRAM 200through a memory controller 330. The memory controller 330 includes anaddress bus coupled to the address bus 214 (FIG. 7) to couple rowaddresses and column addresses to the SDRAM 200. The memory controller330 also includes a control bus that couples command signals to thecontrol bus 206 of the SDRAM 200. The external data bus 258 of the SDRAM200 is coupled to the data bus of the processor 302, either directly orthrough the memory controller 330.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

1-22. (canceled)
 23. A control circuit for applying a select signal to agate electrode of a pass transistor responsive to an access signal, thetransistor having gate-source terminals coupled between first and secondterminals, the control circuit comprising: a shunt transistor having agate terminal and a pair of source-drain terminals coupled between thefirst terminal and the gate electrode of the pass transistor; and acontrol transistor having a gate terminal coupled to receive the accesssignal, the control transistor having a pair of source-drain terminalscoupled between the first terminal and the gate terminal of the shunttransistor.
 24. The control circuit of claim 23, further comprising afirst switching transistor having a gate terminal coupled to receive theaccess signal, and a pair of source-drain terminals coupled between asupply voltage and the gate terminal of the control transistor.
 25. Thecontrol circuit of claim 24, further comprising a second switchingtransistor having a gate terminal coupled to receive a compliment of theaccess signal, and a pair of source-drain terminals coupled between thesupply voltage and the gate terminal of the shunt transistor. 26-41.(canceled)
 42. A method of applying a test voltage to each of aplurality of circuit wells fabricated in a semiconductor substrate of anintegrated circuit and isolated from each other, the method comprising:applying a test voltage to the integrated circuit through an externallyaccessible terminal; and coupling the externally accessible terminal toone of the circuit wells while isolating the externally accessibleterminal from the other circuit wells regardless of the magnitude andpolarity of the test voltage.
 43. The method of claim 42 wherein the actof coupling the externally accessible terminal to one of the circuitwells while isolating the externally accessible terminal from the othercircuit wells comprises: providing first and second transistorsfabricated in respective wells of the substrate for each of the circuitwells, the first and second transistors having first source-drainregions interconnecting to each other and second source-drain regionscoupled to the wells in which they are fabricated; coupling the firstand second transistors for each of the circuit wells between theexternally accessible terminal and the respective circuit well; applyingsignals to gate electrodes of the first and second transistors for theone circuit well that causes the first and second transistors to turnon; and applying signals to gate electrodes of the first and secondtransistors for the other circuit wells that causes the first and secondtransistors to turn off.